This disclosure relates to a coolant plate assembly having integral reactant flow fields, an integral cooler and an integral thermal dam used as an end-cooler assembly in a cell stack assembly.
One type of fuel cell is a phosphoric acid fuel cell (PAFC). One particular configuration of PAFC includes a cell stack assembly (CSA) having 376 cells. Coolers are arranged between every eight cells. Additionally, a cooler is provided at either end of the CSA between the end cells and their adjoining pressure plates. Thus, there are forty-eight total coolers used in this particular configuration.
A coolant fluid, such as water, is provided to the coolers as liquid. The coolant fluid exits the coolers as a two-phase liquid-steam mixture. It is desirable to design a coolant distribution system to produce nearly equal flow to every cooler. This design task is difficult to achieve because the pressure drop across the coolers is a function of coolant flow rate and the quantity of steam produced. The coolers on the interior of the CSA receive waste heat from the cells arranged on either of its sides. The end coolers only receive waste heat from the cells on one of its sides, since the pressure plate is located on its other side. Thus, less steam is produced in the end coolers compared to the interior coolers, which results in increased coolant flow in the end coolers. Accordingly, the end coolers will have a lower temperature, which can cause the adjoining cells to operate at a less than desired temperature. Undesirably low temperatures reduce the cells' tolerance to the carbon monoxide typically present in reformed natural gas fuel, which reduces cell efficiency.
Thermal dams have been used in PAFC CSA between the end coolers and their adjacent cell to increase the temperature of the adjacent cells. One type of thermal dam is formed by laminating a 5.0 mil (0.127 mm) fluorinated ethylene propylene (FEP) polymer film between a pair of 19.7 mils (0.5 mm) carbonized substrates to provide a laminate. The laminate is secured between a pair of 90 mils (2.29 mm) preforms constructed from 89% flake graphite and 11% FEP preforms under a pressure of 200 psi (1400 kPa) at 343° C. (650° F.) and cooled under compression to below 200° C. (392° F.). The thermal dam is machined to a final thickness of 170 mils (4.3 mm) after lamination.
A carbonized substrate is a substrate made by a procedure similar to that outlined in U.S. Pat. No. 4,851,304 issued to Toray except that the high temperature graphitization step is eliminated. The thermal conductivity of one type of substrate carbonized at about 1550° F. or 845° C. is about 0.1 Btu/hr-ft-° F. (0.17 W/m-° K), which is about 1/7 that of a graphitized substrate. The total temperature increase across the thermal dam is adjusted by varying the carbonization temperature of the carbonized substrate or by varying the thickness of the carbonized substrate. One type of PAFC thermal dam uses two 20 mils (0.50 mm) thick carbonized substrates. This results in a temperature increase of about 30° F. (17° C.) across the thermal dam at rated power which raises the temperature of the cell adjacent to the cooler by this amount.
A typical end-cell assembly is quite complicated. The perimeter of each of the thermal dam, the cooler and the pressure plate is wrapped in polytetrafluoroethylene (PTFE) tape to prevent acid absorption. The perimeters of the end-cell assembly components are aligned with one another. The graphite-FEP flow field plate is bonded to a graphitized separator plate. The flow field plate bonded to a graphitized separator, a molded cooler assembly, the thermal dam and a pressure plate, which are separate, discrete components from one another. Manufacturing and assembling these components is time consuming and expensive. The graphitized separator plate is arranged between the thermal dam and the flow field plate. Interfaces between the components are painted with a fluoroelastomer to create a seal against acid and gas penetration.
Exemplary prior art system are disclosed in U.S. Pat. Nos. 4,728,585; 4,929,517; 5,558,955; and 6,050,331, which are incorporated by reference.